The present invention relates to a method for processing received signals having been transmitted via a transmission channel and also relates to a corresponding device. In particular, the present invention is directed to a method and corresponding device for an improved estimation of the channel impulse response in TDMA systems.
Recently, mobile radio telecommunication systems have widely spread. Such mobile radio telecommunication systems operate for example according to a commonly agreed standard, like for example the GSM standard. According to GSM standard, data transmission is performed according to a method of time divisional multiple access (TDMA). The TDMA transmission principle specifies that data are transmitted from a transmitter to a receiver and vice versa only during respectively specified time slots of frames.
Data transmission in such telecommunication systems substantially relies on digital data transmission. However, between a mobile radio transceiver device as a subscriber terminal (hereinafter: mobile station MS) and a stationary radio transceiver device as a serving radio network element (hereinafter: base station BS) data have necessarily to be transmitted as analogue data via the air interface Um.
Consequently, data transmitted by the mobile station MS are received via a base station antenna means of a reception device of the base station BS as analogue data. In the course of the further processing of the thus received data by the reception device, the analogue data are analog to digital converted, i.e. passed through an A/D converter means. At the next stage of the processing, the obtained digital data are supplied to an equalizer means for being equalized. The thus obtained output data are then supplied to a channel codec means for coding/decoding the data. At the reception side, decoding is performed to separate received speech data from associated signaling data.
Particular attention in the course of this processing has to be paid to the equalizing of the received data, since the equalizing is required to reconstruct, at the reception side, the transmitted signal from a mixture of received signals.
For example, assuming a situation in a radio telecommunication network with a base station BS and only a single mobile station MS present in the radio coverage area of the base station. Then, a signal s transmitted from the mobile station MS may reach the base station BS directly via line of sight at a time s(t). However, the same signal s may be deflected by, e.g., a building, a mountain or the like present in the environment. Thus, the same signal may reach the base station BS at a later point of time s(t+T), and is thus superposed to the signal s(t). Due to the delay T, both received signals are no longer in phase with each other. Moreover, the delayed signal s(t+T) may even be more attenuated than the signal s(t) due to the longer transmission path. Thus, the signal received by the base station BS and originating from the mobile station MS is distorted. Now, assuming that another mobile station MSxe2x80x2 is additionally present, then signals sxe2x80x2(txe2x80x2), sxe2x80x2(txe2x80x2+Txe2x80x2) are additionally received by the base station BS, which may lead to interference between the respective transmitted data symbols (intersymbol interference).
Therefore, an equalizer means has to reconstruct (detect) the initially transmitted signal s(t) and/or sxe2x80x2(txe2x80x2) from the received mixture of signals s(t), s(t+T), sxe2x80x2(txe2x80x2), sxe2x80x2(txe2x80x2+Txe2x80x2).
The thus reconstructed (or detected) signal is required to be as similar to the originally transmitted signal as possible. This reconstruction is therefore a main concern when designing equalizers, e.g. for use in a reception device of a base station BS.
Hitherto, in equalizers of reception devices used in mobile telecommunication systems, the channel impulse response (CIR) is estimated, and the thus estimated channel impulse response is used to detect, i.e. to reconstruct the transmitted data symbols.
The estimated channel impulse response is usually based on the received samples of the stream of data symbols. Therefore, it is an estimate of the actual (observed) channel impulse response. However, an error in the channel impulse response leads to a degraded performance when detecting/reconstructing transmitted data symbols from received data symbols.
In a previous solution, the above described estimated channel impulse response is assumed to be ideal, thereby disregarding errors, and it is used as it is for data symbol reconstruction. However, this solution was unsatisfactory due to the error in the observed channel impulse response being not taken into account.
Another known solution for improving the performance of an equalizer is disclosed in document U.S. Pat. No. 5,251,233 by Labedz et. al., assigned to Motorola Inc. The basic idea described therein is to estimate the energy of taps in the impulse response. Those taps having an energy below a predetermined threshold level are zeroed in order to reduce noise in the estimated channel impulse response. This previously proposed method may be applied to complex taps or real taps, i.e. may be performed separately for real and imaginary taps. The method disclosed in the Labedz patent improves (i.e. reduces) the bit error rate BER in the reproduced detected signal (which is an indication for the quality of a receiver) in situations where the channel interference ratio C/I or C/(I+N), respectively, with N being a noise signal superposed to an interfering signal, is low and the channel has a poor quality. The method is also beneficial in channels having a short impulse response. (The term taps refers to coefficients of each respective delay element of the corresponding FIR model for the used circuitry. Thus, based on the tap values, poles and zeroes of the transmission function may be calculated.)
However, under good channel conditions, the method as proposed by Labedz et. al. even degrades the receiver performance which may be attributable to the zeroing of amplitude coefficients of certain taps.
Additionally, document EP-A-0 701 334 A2 proposes a method and device to measure an impulse response of a radio channel which are respectively based on a correlation method which is performed twice.
However, using such a method, the estimation of the channel impulse response function to be used in an equalizer is somewhat delayed due to the double estimation cycle being performed.
Another approach has been conceived by NORTEL. This approach uses channel impulse response estimation methods which are based on least square error (LSE) methods.
U.S. Pat. No. 5,379,324 teaches to calculate noise variance in order to create more accurately an information signal.
The publication xe2x80x9cTraining Sequence Design for Adaptive Equalization of Multi-User Systemsxe2x80x9d by G. Caure, U. Mitra, in CONFERENCE RECORD OF THE THIRTY-SECOND ASILOMAR CONFERENCE ON SIGANLS, SYSTEMS AND COMPUTERS, November 1-4, 1998, Vol. 2, pages 1479-1483, XP002112369 1998, Piscateway, N.J., USA, IEEE, discloses the features of the preamble of the present independent claims 1 and 8, respectively.
Consequently, it is an object of the present invention to provide a method and device for processing received signals having been transmitted via a transmission channel which are free from the above drawbacks and which are particularly suitable for quickly estimating a most reliable channel impulse response function.
According to the present invention, this object is achieved by a method for processing received signals having been transmitted via a transmission channel as defined in claim 1.
Still further, according to the present invention, this object is achieved by a signal processing device for processing received signals having been transmitted via a transmission channel as defined in claim 8.
Advantageous further developments are as set out in the respective additional dependent claims.
According to the present invention, it can be dispensed with modifying an estimated channel impulse response function as it is the case in some of the above mentioned previous solutions.
According to the proposed invention the channel parameters and transmitted data and/or symbols and other quantities derived therefrom is in matrix representation. Normally, a processing of such matrixes increases the complexity of the processing when estimating the channel impulse response parameters. However, according to the proposed solution the matrix representation could be reduced to diagonal matrixes (having values differing from zero only in their diagonal), so that the processing load could be kept small, thereby resulting in a fast estimation process for the channel impulse response function. According to the inventors experiences in connection with applying the invention in a system operated according to GSM, for implementing the invention only a 7xc3x977 matrix representation of the respective quantities is required. Nevertheless, in a preferred realization, in which also a time of arrival information (TOA) of respective signals is obtained in a pre-processing based on an obtained preliminary channel impulse response function, the matrix representation may even be reduced to a 5xc3x975 matrix size.
The present invention has been found to be implementable in all channels as specified by the GSM 5.05 recommendation. The receiver performance could be improved in all such channels. Moreover, the improvement of received performance was observed in all channel interference situations, i.e. under various C/(I+N) conditions. In particular, the observed bit error rate (BER) as a measure for the receiver quality could be reduced by at least 15% as compared to a case in which the present invention has not been implemented.
Moreover, the present invention may easily be applied to and combined with other concepts for channel impulse response improvements, as for example those concepts described in the international patent application no. PCT/EP98/04562 of the same applicant (not yet published).
Preferred embodiments of the present invention are described herein below in detail with reference to the accompanying drawings.